1. Field of the Invention
The present invention relates generally to a method for transmitting and receiving a control message in a mobile communication system, and in particular, to a method for transmitting and receiving a control message between a broadcast/multicast-service center (BM-SC) and a user equipment (UE) in a mobile communication system supporting a Multimedia Broadcast/Multicast Service (MBMS).
2. Description of the Related Art
Currently, due to the development of the communication industry, a service provided in a Code Division Multiple Access (CDMA) mobile communication system is developing into a multicasting multimedia communication service capable of transmitting a large volume of data such as packet data and circuit data, as well as voice data. In order to support the multicasting multimedia communication service, active research is being carried out on a Broadcast/Multicast Service in which a service is provided from one data source to a plurality of user equipments (UEs). Generally, the Broadcast/Multicast Service can be classified into a Cell Broadcast Service (CBS), which is a message-oriented service, and a Multimedia Broadcast/Multicast Service (MBMS) supporting multimedia data such as real-time image and voice, still image, text, etc.
FIG. 1 is a block diagram schematically illustrating a network configuration for providing an MBMS service in a mobile communication system. Referring to FIG. 1, a broadcast/multicast-service center (BM-SC) 110 is a source for providing an MBMS stream. The BM-SC 110 schedules a stream for an MBMS service and delivers the scheduled MBMS stream to a transit network (N/W) 120. The transit network 120 is a network existing between the BM-SC 110 and a serving GPRS (Global Packet Radio System) support node (SGSN) 130, and delivers an MBMS stream provided from the BM-SC 110 to the SGSN 130. The SGSN 130 includes a gateway GPRS support node (GGSN) and an external network, and it will be assumed herein that there are a plurality of UEs desiring to receive the MBMS service at a particular time, for example, a UE#1 161, a UE#2 162, and a UE#3 163 located in a Node B#1 (cell#1 160), and a UE#4 171 and a UE#5 172 located in a Node B#2 (cell#2 170).
The SGSN 130, receiving the MBMS stream from the transit network 120, controls a function of controlling an MBMS-related service for subscribers, or UEs, desiring to receive an MBMS service, for example, controlling an MBMS-related service of managing MSMS service accounting-related data of each of the subscribers and selectively transmitting MBMS data to a particular radio network controller (RNC) 140. Herein, the “Node B” will be depicted as a “cell,” for convenience. As usual, the Node B may manage only one cell, or manage a plurality of cells.
The SGSN 130 should perform selective MBMS data transmission to the RNC 140. The RNC 140 should also perform selective MBMS data transmission to its associated cells. Accordingly, the SGSN 130 should include a list of RNCs receiving the MBMS service, and the RNC 140 should include a list of cells receiving the MBMS service. Therefore, the RNC 140 can later provide an MBMS service to the cells in the list stored therein. The RNC 140 controls a plurality of cells, and transmits MBMS data to a cell in which a UE requesting an MBMS service is located, among its own cells. In addition, the RNC 140 controls a radio channel established to provide the MBMS service, and manages information on the MBMS service using an MBMS stream provided from the SGSN 130. Though not illustrated in FIG. 1, a hone location register (HLR) is connected to the SGSN 130 and performs subscriber authentication for an MBMS service.
In order to provide a particular MBMS service, basic information on the particular MBMS service is delivered from a network to UEs. Also, a list of UEs desiring to receive the particular MBMS service among the UEs receiving the basic information on the particular MBMS service is delivered to the network and managed therein.
Upon receiving the list of UEs desiring to receive the particular MBMS service, the network pages the UEs and establishes a radio bearer for providing the MBMS service. After establishing the radio bearer to the UEs, the network starts transmission of MBMS data for the particular MBMS service through the established radio bearer. The MBMS data is transmitted from a BM-SC included in the network, and delivered to the UEs desiring to receive the particular MBMS service. In order to protect the MBMS data delivered from the BM-SC to the UEs, a separate protection procedure should be performed. That is, a procedure for performing authentication and authorization on users or terminals and encrypting the contents for an MBMS service should be performed.
FIG. 2 is a diagram illustrating a procedure for performing encryption before transmission, in order to protect MBMS data in a mobile communication system providing an MBMS service. More specifically, FIG. 2 illustrates a procedure in which a BM-SC encrypts MBMS data and transmits the encrypted MBMS data to a UE in a mobile communication system supporting the MBMS service.
Referring to FIG. 2, an encryption operation on MBMS data for an MBMS service and a user authentication operation are achieved in a BM-SC 202. A UE 201 is a terminal desiring to receive the MBMS service. The BM-SC 202 protects MBMS data for the MBMS service by encryption, and transmits the protected MBMS data to the UE 201 in step 203. Herein, every MBMS data packet transmitted from the BM-SC 202 includes a key identifier (ID) indicating a primary encryption key (or main encryption key), and a random seed value. The key ID of a primary encryption key and the random seed value are used for acquiring a secondary encryption key (or auxiliary encryption key), which is directly used encrypting the MBMS data. That is, the primary encryption key and the random seed value are used as input values of a function for calculating a secondary encryption key used for actually encrypting MBMS data.
Therefore, the UE can calculate a secondary encryption key by acquiring the input values included in the received MBMS data packet. By acquiring the secondary encryption key, the UE can decrypt the MBMS data, which was encrypted before being transmitted. The primary encryption key and the random seed value are transmitted without being encrypted.
The primary encryption key is a key shared by the UE 201 and the BM-SC 202, and before an updated primary encryption key is applied, a procedure in which the UE 201 and the BM-SC 202 distribute the key is performed. This procedure can happen periodically or on an event-by-event basis. Also, a secondary encryption key used for actually encrypting MBMS data can be updated periodically. The update of the secondary encryption key is shorter in period than the update of the primary encryption key.
A current 3rd Generation Partnership Project (3GPP) standard has not specified an update period of the keys. However, including the key ID of the primary encryption key and the random seed value in MBMS data during every transmission of the MBMS data causes unnecessary overhead. That is, including the key ID of the primary encryption key and the random seed value in MBMS data during transmission reduces the amount of actually transmitted MBMS data or causes overhead in which data is unnecessarily repeatedly transmitted due to the encryption.
For example, assuming that the key ID is expressed with 6 bits and the random seed value is expressed with 128 bits, although a key value used for actually encrypting data has not been changed, every MBMS data transmits 134 bits (key ID+random seed value), causing unnecessary overhead. The overhead results in a reduction in an amount of MBMS data that can be actually transmitted.